The present invention relates to the synthesis of polycarbonate by melt polymerization. More particularly, the present invention describes methods and systems for polymerization of polycarbonate as a spray to facilitate volatilization of inhibitory reaction byproducts and thereby increase the rate at which polymerization occurs.
Polycarbonates, such as bisphenol A polycarbonate, are typically prepared either by interfacial or melt polymerization methods. The reaction of a bisphenol such as bisphenol A (BPA) with phosgene in the presence of water, a solvent such as methylene chloride, an acid acceptor such as sodium hydroxide, and a phase transfer catalyst such as triethylamine is typical of the interfacial methodology. The phase transfer catalyst improves the solubility of the phenolate in the organic phase and greatly speeds the reaction. Alternatively, polycarbonates may be made by ester exchange using, for example, dimethyl carbonate or diphenyl carbonate. The reaction of bisphenol A with a source of carbonate units such as diphenyl carbonate at high temperature in the presence of a catalyst such as sodium hydroxide is typical of currently employed melt polymerization methods. Each method is practiced on a large scale commercially and each presents significant drawbacks.
The interfacial method for making polycarbonate has several inherent disadvantages. First, it is a disadvantage to operate a process which requires phosgene as a reactant due to known safety concerns. Second, because the process requires using large amounts of an organic solvent, expensive precautions must be taken to guard against any adverse environmental impact. Third, the interfacial method requires a relatively large amount of equipment and capital investment. Fourth, the polycarbonate produced by the interfacial process may be prone to inconsistent color, higher levels of particulates, and higher chloride content, which can cause corrosion.
The melt method, although obviating the need for phosgene or a solvent such as methylene chloride, requires high temperatures and relatively long reaction times. As a result, by-products may be formed at high temperature, such as the products arising by Fries rearrangement during polymerization. Fries rearrangement gives rise to undesired and uncontrolled polymer branching which may negatively impact the polymer's flow properties and performance. The melt method further requires the use of complex processing equipment capable of operation at high temperature and low pressure, and capable of efficient agitation of the highly viscous polymer melt during the relatively long reaction times required to achieve high molecular weight.
Using salicyl carbonate as a substitute for diphenylcarbonate allows for significantly faster reaction rates and more favorable equilibrium in melt polymerization reactions. For example, it has been reported that melt polycarbonate can be formed under relatively mild conditions by reacting a bisphenol such as BPA with the diaryl carbonate formed by reaction phosgene with methyl salicylate (U.S. Pat. No. 4,323,668 and WO99/47580). Also, polycarbonates comprising salicylic ester derivatives may be generated to have excellent optical tone and/or color hue (see e.g., WO00/63274; WO98/45246; JP10036497; JP10101786; JP10101787; and JP11302228) and thermal stability (JP2001158821). Still, relatively high levels of transesterification catalysts are generally required to produce high molecular weight polycarbonate using reported methods. Another significant drawback is that the salicylate formed as a byproduct of the reaction inhibits the reaction from proceeding.
What is needed is a simple method to promote the formation of polycarbonate using diaryl carbonates such as bis(methyl salicyl) carbonate that allow for mild reaction conditions while promoting the removal of inhibitory byproducts. Ideally, the method can be incorporated into the production of commercially suitable polycarbonate using existing equipment and protocols. Also ideally, the method will allow polymerization using conditions that are relatively mild (i.e. <400° C.) and for short time periods to minimize the generation of Fries byproduct and/or the breakdown of the polycarbonate product while still allowing for efficient volatilization of inhibitory salicylate byproduct.